Trying to understand a formula to calculate elapsed time in a specific time period

Hello Ivirden, that looks like one of mine....

In that formula the fractional amounts represent times, 3/4=18:00, 1/3 = 08:00 and 5/12 represents the length of the working day 18:00-8:00=10:00.

It's perhaps easier (and more flexible) if you put the day start (08:00) and day end (18:00) in two cells and reference those, e.g. using J2 and J3 that would be:

=(NETWORKDAYS(D1,E1)-1)*(J$3-J$2)+IF(NETWORKDAYS(E1,E1),MEDIAN(MOD(E1,1),J$3,J$2),J$3)-MEDIAN(NETWORKDAYS(D1,D1)*MOD(D1,1),J$3,J$2)

This particular formula is more complex because it still calculates correctly if the start or end times are outside the working hours, e.g. if start time might be 06:00 or end time on a Saturday.

If your start and end times will always be between J2 and J3 times (on Monday to Friday) then this formula will be sufficient:

=(NETWORKDAYS(D1,E1)-1)*(J$3-J$2)+MOD(E1,1)-MOD(D1,1)

Both formulas essentially count the number of working days and multiply that by the number of hours in a working day - and then make an adjustment for the start and end times, with the former that's clearly more complex........

...I can give you more detail if you want.....